1. Field of the Invention
The present invention relates to an ink ejection device for forming images on a recording medium by electing ink droplets from nozzles according to printing commands.
2. Description of the Related Art
Non-impact type printing devices have recently taken the place of conventional impact type printing devices and are holding an ever-growing share of the market. Of these non-impact type printing devices, ink-ejecting type printing devices have the simplest operation principle, but are still capable of effectively and easily performing multi-gradation and color printing. Of these devices, a drop-on-demand type for ejecting only ink droplets which are used for printing has rapidly gained popularity because of its excellent ejection efficiency and low running cost.
A shear mode type printer using a piezoelectric actuator is one of the drop-on-demand types. Such a printer is disclosed in U.S. Pat. No. 4,879,568. One example of such type of printer is shown in FIGS. 1(a) and 1(b) in which FIG. 1(a) is a cross-sectional view taken along line Axe2x80x94A in FIG. 1(b) an FIG. 1(b) is also a cross-sectional view taken along line Bxe2x80x94B in FIG. 1(a).
As shown in FIGS. 1(a) and 1(b), the shear mode type ink ejection device 600 includes a bottom wall 601, a ceiling wall 602, and elongated shear mode actuator walls 603 sandwiched therebetween. Each actuator wall 603 includes a lower wall 607 adhesively attached to the bottom wall 601 and an upper wall 605 adhesively attached to the ceiling wall 602. The upper and lower walls 605, 607 are polarized in the directions indicated by arrows 609, 611, respectively. Alternating pairs of actuator walls 603 form in alternation ink channels 613 therebetween or spaces 615, which are narrower than the ink channels 613.
Electrodes 619 and 621 are provided on both side surfaces of each actuator wall 603. Specifically, the electrode 619 is provided in the ink channel 613 and the electrode 621 is provided in the space 615. The electrode 621 is also provided on the outer side surface of each of the two outermost actuator walls 603. The electrode 619 is covered by an insulating layer (not shown) to insulate it from the ink. The electrodes 621 are connected to ground 623. The electrodes 619 are connected to a control unit 625 in a form of a silicon chip which applies voltages (driving signals) to the electrodes 619 as will be described later.
A nozzle plate 617 is fixedly secured to one end of the actuator walls 603. The nozzle plate 617 is formed with nozzles 618 at positions corresponding to the ink channels 613. An ink supplying source (not shown) is connected to the other end of the actuator walls 603 through a manifold 626. The manifold 626 includes a front wall 627 formed with openings in positions corresponding to the ink channels 613, and a rear wall 628 for sealing the space between the bottom wall 601 and the ceiling wall 602. Ink from the ink supplying source is supplied to the manifold 626 or common ink chamber and distributed into the respective ink channels 613. The front wall 627 prevents ink from the manifold 626 from entering the spaces 615.
To eject droplets, a voltage from the control unit 625 is applied to the electrode 619 of each ink channel 613. Pairs of the actuator walls 603 deform outward by the piezoelectric shear effect so that the volume of each ink channel 613 increases. In the example shown in FIG. 2, when a voltage E volts is applied to the electrode 619c of the ink channel 613c, an electric field is developed in the actuator wall 603e in the direction indicated by the arrow 631, and an electric field is developed in the actuator wall 603f in the direction indicated by the arrow 632. Because the electric field directions 631 and 632 are at right angles to the polarization direction 609, 611, the actuator walls 603e, 603f deform outward to increase the volume of the ink channel 613c by the piezoelectric shear effect, resulting in a decrease in the pressure in the ink chamber 613c, including near the nozzle 618c. 
Application of the voltage E(V) is maintained for a duration of time T, during which time ink is supplied from the ink supplying source. A pressure wave occurring when the ink is supplied from the ink supplying source propagates in the lengthwise direction of the ink channel 613c. The duration of time T corresponds to a duration of time required for the pressure wave to propagate once in the lengthwise direction of the ink channel 613c. The duration of time T (hereinafter referred to as xe2x80x9cpressure wave propagation timexe2x80x9d) can be calculated by the following formula:
T=L/a
wherein L is the length of the ink channel 613; and
a is the speed of sound through the ink filling channel 613c. 
Theories on pressure wave propagation teach that at the moment the duration of time L/a elapses after the application of the voltage, the pressure in the ink channel 613c inverts to a positive pressure. The voltage application to the electrode 619c of the ink channel 613c is stopped in timed relation with this pressure inversion so that the actuator walls 603e, 603f revert to their initial shape shown in FIG. 1(a).
The pressure generated when the actuator walls 603e, 603f return to their initial shape is added to the inverted positive pressure so that a relatively high pressure is generated in the ink channel 613c. This relatively high pressure ejects an ink droplet 26 from the nozzle 618c. 
In the ink ejection device 600 of the type described above, a dot formed by continuously ejected two or more ink droplets in response to one-dot print command must have an increased density than a dot formed by a single ink droplet. However, when the continuously ejected droplets join together during the flight time toward the recording medium, the density of the dot printed on the recording medium is not as high as it is expected. Because a major part of the joined droplet is absorbed in the recording medium. In this case, the printed dot is almost as large as the dot printed by ejecting a single droplet. However, the print density does not increase despite a large amount of ink consumption.
In view of the foregoing, it is an object of the present invention to provide an ink ejection device capable of forming a high density dot with two or more ink droplets that are continuously ejected one from another from the same nozzle.
It is another object of the present invention to provide a method for driving such an ink ejection device.
These and other objects of the present invention will be attained by an ink ejection device including a nozzle plate formed with nozzles from which ink is ejected; walls including side walls, a ceiling wall and a bottom wall; and a control unit that drives the actuator in response to a one-dot printing command commanding to print a one dot image on a recording medium. The walls define an ink channel. The ink channel has a volume filled with ink. The nozzle plate is attached to one end of the ink channel. Ink is supplied to the ink channel from another end of the ink channel. The side walls are made from a piezoelectric material and serve as an actuator that applies pressure wave vibrations to the ink in the ink channel. The actuator successively ejects a plurality of ink droplets to print one dot image on the recording medium. The actuator is driven to eject the plurality of ink droplets so that at least two ink droplets ejected from a nozzle do not join together before reaching the recording medium. The plurality of ink droplets form print dot images on the recording medium in a positionally offset relation.
With the ink ejection device thus constructed, the plurality of ink droplets do not join together during their flight times. Therefore, the ink droplets are successively deposited onto the recording medium. Because the ink ejection device moves relative to the recording medium during printing. the ink droplets are deposited in a positionally offset relation. That is, the ink droplets are not completely separately deposited on the recording medium but partially overlapped to form a single dot image. As such, the size of the dot image is enlarged when compared with a dot image formed by a single ink droplet. Thus, it is capable of forming a high density image.
Preferably, the actuator successively ejects two ink droplets to print one dot image on the recording medium. In this case, the control unit drives the actuator at a particular timing so that two ink droplets do not join together.
The control unit drives the actuator to first increase the volume of the ink channel and to then decrease the volume of the ink channel to eject each of the plurality of ink droplets. The control unit drives the actuator for a duration of time T corresponding to a time required for the ink pressure wave vibrations to propagate one way through the ink channel to eject an n-th ink droplet of the plurality of ink droplets where n is an integer equal to or greater than two. When the volume of the ink chamber is increased, the pressure inside the ink chamber decreases, allowing ink to flow into the chamber. After the duration of time T required for pressure wave vibrations in the ink to propagate once across the length of the ink channel, the control unit drives the actuator to decrease the volume of the ink chamber. The pressure inside the ink chamber becomes relatively high, causing ink to be ejected from the corresponding nozzle.
The control unit drives the actuator to eject an (n+1)th ink droplet following the n-th ink droplet after an interval of a time from the ejection of the n-th ink droplet. The interval of the time is defined by 0.7 to 1.6 times as long as the duration of time T. The control unit drives the actuator for a predetermined duration of time to eject the (n+1)th ink droplet. The predetermine duration of time is unequal to the duration of time T when the interval of time is also equal to the duration of time T.
When ejecting the (n+1)th ink droplet, the control unit drives the actuator for a duration of time substantially equal to:
1.7 times as long as the duration of time T when the predetermined duration of time is equal to 0.7 times as long as the duration of time T;
0.8, 1.0, 1.4 or 1.7 times as long as the duration of time T when the predetermined duration of time is equal to 0.8 times as long as the duration of time T;
0.8 times as long as the duration of time T when the predetermined duration of time is equal to 0.9 or 1.0 times as long as the duration of time T;
0.5 or 0.6 times as long as the duration of time T when the predetermined duration of time is equal to 1.2 times as long as the duration of time T;
0.5 or 1.0 times as long as the duration of time T when the predetermined duration of time is equal to 1.3 times as long as the duration of time T;
1.0 times as long as the duration of time T when the predetermined duration of time is equal to 1.4 times as long as the duration of time T;
0.5, 0.7 or 1.0 times as long as the duration of time T when the predetermined duration of time is equal to 1.5 times as long as the duration of time T; and
1.7 times as long as the duration of time T when the predetermined duration of time is equal to 1.9 times as long as the duration of time T.
The control unit may drive the actuator for a duration of time in a range of xc2x15% of the duration of time as mentioned above.
The control unit drives the actuator after ejection of the plurality of ink droplets to cancel the pressure wave vibrations remaining in the ink of the ink channel.
According to another aspect of the invention, the control unit drives the actuator for a first duration of time to eject an n-th ink droplet of the plurality of ink droplets, the control unit drives the actuator for a second duration of time to eject an (n+1)th ink droplet following the n-th ink droplet, the (n+1)th ink droplet being ejected after an interval of a time from the ejection of the n-th ink droplet. In this case, at least one of the first duration of time, the second duration of time, and the interval of the time are set to be unequal to a duration of time T corresponding to a time required for the ink pressure wave vibrations to propagate one way through the ink channel.
At least one of the first duration of time, the second duration of time, and the interval of the time is unequal to an odd multiple of the duration of time T.
According to still another aspect of the invention, there is provided an ink ejection device, including: a nozzle plate formed with nozzles from which ink is elected; walls including side walls, a ceiling wall and a bottom wall; and a control unit that outputs a plurality of drive signals to the actuator in response to a one-dot printing command commanding to print a one dot image on a recording medium. The walls define an ink channel. The ink channel has a volume filled with ink. The nozzle plate is attached to one end of the ink channel, and ink is supplied to the ink channel from another end of the ink channel. The side walls serve as an actuator that applies pressure wave vibrations to the ink in the ink channel. The actuator generating in the ink of the ink channel a pressure wave vibration having a peak when each of the plurality of drive signals is applied to the actuator, causing an ink droplet to eject. The control unit outputs the plurality of drive signals at timings that do not allow the peak of the pressure wave vibration to be in coincidence with an existing peak of the pressure wave vibration occurring in the ink of the ink channel. The actuator first increases the volume of the ink channel and then decreases the volume of the ink channel to eject each of the plurality of ink droplets. Preferably, the control unit applies voltage signals to the actuator to eject the plurality of ink droplets. The voltage signals have the same level.